Various liquid or semi-liquid compositions, such as coating and adhesive compositions frequently contain flammable components which can cause such compositions to undergo combustion if exposed to a source of ignition such as an electrostatic discharge. Because of various federal and provincial regulations relating to the transportation of flammable materials, flammable liquids are generally transported and stored in steel or other rigid metal containers. These metal containers are generally durable and are readily reusable or recyclable provided that they have not been chemically attacked, soiled or encrusted by the liquid or semi-liquid material contained therein. Because of their durability, and also because of the relatively high costs associated with the fabrication and disposal of suitable metal containers for transporting and storing flammable liquid or semi-liquid compositions, it is highly desirable to reuse such containers. Generally, however, it is not safe, practical and cost effective to reuse metal containers which have been in direct contact with materials which either cause corrosion or otherwise chemically attack the walls of the metal container, or which soil, contaminate or otherwise form a tenacious union with the walls of the container. Metal containers which have been corroded or otherwise chemically attacked can lose both structural and functional utility, thereby creating the risk of a potentially hazardous spill or leak. Reuse of such containers would entail careful and costly inspection, and repair, or both, and would not completely eliminate the risks associated with the reuse of chemically attacked containers. Reuse of soiled containers such as those having solids encrusted on the walls thereof would generally be impractical because of the expense associated with cleaning and reconditioning such containers.
In order to facilitate reuse of metal containers for shipping and storing flammable liquid or semi-liquid materials, problems caused by direct contact of the liquid or semi-liquid material with the walls of the metal container have been effectively eliminated by using plastic inserts or liners which are impermeable to the materials contained therein. However, a very serious problem arises when plastic liners are used for storing flammable materials. This problem relates to the tendency of most plastics to easily acquire an electrostatic charge. An electrostatic charge can be easily generated on the surface of most plastics by creating friction between the plastic and another material. Once a plastic liner has acquired an electrostatic charge, an abrupt electrical discharge or spark can occur between the surface of the plastic liner and an object in close proximity which has a different electrical potential. The electrical spark in turn can ignite flammable vapors and/or liquids nearby resulting in a fire hazard. Because the liner is disposable, whereas the metal pail is typically recyclable, it is common to remove the liner from the pail immediately after the contents of the liner has been used. Removal of a non-static dissipative plastic liner from a metal pail can easily create electrical sparks, especially if the relative humidity is low. Such electrical spark can, especially in poorly ventilated areas, ignite volatile flammable components emanating from residual material left on the walls of the liner. Accordingly, such liners must generally meet certain static dissipative criteria. For example, the National Fire Protection Association (NFPA) specifies that such liners be static dissipative as determined by ASTM D-257 "Surface Resistivities" procedure and by the Static Decay Test described in Federal Test Method Standard 101C, Method 4046, "Electrostatic Properties of Materials" Specifically, the plastic container liners should have a surface resistivity of between 10.sup.5 and 10.sup.11 ohms/square, and should have a static decay of less than 0.5 seconds @10% cutoff for static decay.
Prior attempts to provide plastic container liners for preventing contact between a metal container and a flammable liquid or semi-liquid composition contained therein, and which exhibit the desired or mandatory static dissipative properties have been directed toward the use of any of various conventional antistatic agents, such as quaternary ammonium compounds, alkyl amines, ethoxylated or propoxylated compounds, and glycerol compound, which are either applied to the surface of the liner or incorporated directly into the plastic used to form the liner. An inherent disadvantage of surface treatments is that they are quickly and easily removed and care must be taken to ensure that they are applied to the entire surface. Incorporating conventional antistatic chemicals directly into the plastic composition used to form the liner also has certain disadvantages. Specifically, antistatic chemicals incorporated into the polymeric plastic composition are intended to continually migrate to the surface and form a static dissipative film thereon. Accordingly, the antistatic additives must exhibit a relatively specific degree of compatibility and mobility in the polymeric composition to migrate to the surface in the desired manner. As a consequence, the amount of antistatic additive which must be added to achieve the desired static dissipative properties is generally critical over a narrow range. This is particularly true for polyolefin compositions, which are presently preferred for use in fabricating the liners because of their excellent chemical resistance and ability to effectively retard diffusion or permeation of the various components contained in a variety of flammable liquid or semi-liquid compositions. Another related disadvantage arises from the fact that subsequent exposure to even moderately high temperatures, after the antistatic chemical additive has been incorporated into the polymeric composition which is used to form the liner, will tend to cause excessive blooming of the antistatic additive from the plastic, which can result in a significant or total loss of static dissipative properties. Such exposure to heat can occur, for example, during thermoforming operations during fabrication of the liner.
A primary object of the invention is to provide a suitable container liner which will serve as an effective barrier for preventing contact between the walls of a metal container and a flammable liquid or semi-liquid composition contained therein, which will permanently achieve the desired static dissipative properties irrespective of exposure to moderately elevated temperatures, and which does not rely on surface treatments which can be easily and inadvertently removed.